Universal single phase motor starting control apparatus

ABSTRACT

A starting circuit for single phase electric motors including both split-phase and capacitor start motors includes a gate controlled solid state switch serially connected to the start winding of the motor. Rectified reference pulses from a pulse transformer are generated to turn on a first transistor to provide gating current for the solid state switch. Initially, when the motor is energized at zero rpm, the pulses are received at the switch after the start winding current passes through the zero current level to gate the switch to conduct each half cycle and energize the start winding however as the motor speeds up, the pulses are received earlier and earlier relative to the start winding current zero cross over until at a selected speed the pulses are received at the switch prior to the start winding current zero cross over with the result that the switch is no longer gated conductive. When this occurs, the voltage across the switch goes high. This voltage is rectified and received at the base of a second transistor adapted to shunt the pulses from the pulse transformer away from the first transistor to lock out the switch with the start winding deenergized. In a first embodiment the pulse transformer is energized by the main winding current to directly employ the phase difference between main winding current and start winding current while in a second embodiment the pulse transformer is energized by line current to directly employ the phase difference between line current and start winding current.

CROSS REFERENCE

The subject matter of this application is also contained in U.S. Ser.No. 492,538 filed of even date herewith.

BACKGROUND OF THE INVENTION

This invention relates generally to single phase electric motor startersand more particularly to a universal motor starter for such motors.

The utilization of solid state switches for motor starting to improvereliability and longevity over conventional electromechanical relays iswell known. Typically a gate controlled solid state switch, such as atriac, is serially connected to the start winding of a motor and isadapted upon initial energization of the motor to be gated into a lowimpedance state thereby permitting current flow in the start winding.After a brief period of time the gating current to the triac isinterrupted causing the triac to go into a high impedance state toeffectively deenergize the start winding. Many different approaches havebeen made, with varying degrees of success, to utilize one or morecharacteristics of the motor to prevent conduction of the triac andhence effect deenergization of the start winding at the optimum moment.For example, as disclosed in General Electric Application Note200.35-3/66 page 16, line current is used to turn on the triac whichdrops out once the current settles down to normal levels. In U.S. Pat.No. 3,414,789 to Prouty main winding current is used to control theconductive state of the triac by means of the voltage across animpedance serially connected to the main winding. In U.S. Pat. No.3,671,830 to Kruper the voltage across the start winding is used tocontrol conduction of the triac through a Schmitt trigger arrangement.In U.S. Pat. No. 3,421,064 to Phillips a control winding, magneticallycoupled to the main winding, develops a voltage vector which is comparedwith a voltage vector developed across a portion of the main windingwith the vector difference used to control the conduction of the triac.In U.S. Pat. No. 3,746,951 to Hohman impedance elements are connectedacross the main winding to monitor motor speed by sensing the relativephase difference between start winding current and applied voltage tocontrol the conductive state of the triac. U.S. Pat. No. 3,777,232 toWoods et al also employs phase angle relationships to control conductionof the triac by comparing the phase difference between main windingcurrent and applied voltage in one embodiment and between main windingcurrent and start winding current in another embodiment. In U.S. Pat.No. 4,307,327 to Streater et al the phase angle between start windingcurrent and line current is used to trigger the triac through a reedswitch disposed in the trigger circuit of the triac.

All of the above approaches suffer from one or more limitations withregard to their usefulness. For example, in several of the aboveincluding the General Electric approach, Prouty and Kruper, variationsin voltage supply and loading effect the motor speed at which the startwinding is deenergized resulting in inconsistent performance. Anotherdisadvantage common to several of the circuits is that they requirespecific tailoring for them to be effective for a given motor. This istrue of Phillips, Streater et al and Hohman. The approach of Woods et alsuffers from a reliability problem since a triac is located in the mainwinding circuit and is adapted to be energized concomittantly with themain winding. The Woods et al circuit is also relatively complex and isinherently expensive due to the many components employed therein.

In addition to the above noted disadvantages, many of the above notedprior art circuits permit reenergization of the start winding undercertain conditions to provide extra torque however in many applications,this can have an adverse effect on reliability and longevity which, asmentioned supra, are two of the main reasons for using triacs. Forexample, in cases where the motor might be subjected to continuousrestarting, particularly under a heavy load such as a stall condition,there is a danger that the junction temperature of the triac could risetoo high and the triac burn out.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a singlephase electric motor starter which does not have the prior artlimitations mentioned above. Another object is the provision of a solidstate motor starter which is reliable yet simple and economical. Yetanother objective is the provision of a motor starter which can be usedwith both split-phase and capacitor start motors and which can be usedwith a whole class of motors without any need for tailoring to aspecific application. Still another objective is the provision of amotor starter which has a positive lockout capability to preventreenergization of the start winding until the motor is deenergized.

Briefly, in accordance with the invention a gate controlled solid stateswitch is serially connected to the start winding of a single phasemotor. A pulse transformer provides a narrow pulse each time, in oneembodiment, the main winding current crosses zero which pulses arerectified and fed to a first transistor disposed in the gate circuit ofthe solid state switch. As long as the main winding current pulses arereceived at the triac after the start winding current crosses zero, thetriac is fired for most of each half cycle. As the speed of the motorincreases, the phase difference between the main winding current andstart winding current decreases until at a point generally between 65and 85% of synchronous speed, depending on the motor, the main windingcurrent pulses are received at the solid state switch before the startwinding zero cross over so that the triac is essentially not turned onfor that half cycle. When that occurs, the high voltage appearing acrossthe triac is rectified and fed to a second transistor which is thenturned on and shunts the pulses away from the first transistor topositively lock out energization of the solid state switch.

In a second embodiment the pulse transformer is adapted to provide thepulses when line current crosses zero and, with the pulses used in thesame manner as in the first embodiment, results in narrower, sharperpulses and less torque loss since the switching of the solid stateswitch occurs closer to the zero cross over of the start windingcurrent.

These and other objects, features and advantages of the presentinvention may be more clearly understood through a consideration of thefollowing detailed description. In the course of this description,reference will be made to the attached drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a preferred control circuit accordingto the invention in which the change with motor speed of the phase anglebetween main winding current and start winding current is directlyutilized to control energization of the start winding;

FIG. 2 is a schematic diagram, similar to FIG. 1, of another preferredcontrol circuit according to the invention in which the change withmotor speed of the phase angle between line current and start windingcurrent is directly utilized to control energization of the startwinding;

FIG. 3 shows current traces for main winding and start winding currentsrelative to the firing of the solid state switch upon initialenergization of the motor. A current trace for line current is alsoshown with reference to the second embodiment.

FIG. 4 shows traces similar to FIG. 3 but shown at a time when pulsesfrom the main winding for the first embodiment and from line for thesecond embodiment reach the triac before the start winding current zerocross over; and

FIG. 5 is a chart showing the phase angle between main winding currentand start winding current and between line current and start windingcurrent versus speed of the motor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As seen in FIG. 1, a single phase induction motor comprising a mainwinding 10 and a start winding 12 is adapted to be energized by a sourceof alternating current 14. A pulse transformer 16 comprises a coil 18connected serially to main winding 10 and a secondary 20 coupled to afull wave diode bridge rectifier 22. Pulse transformer 16 has a toroidwound with thin magnetic tape having a square hysteresis so that pulsesare outputted every time the main winding current crosses zero. Theoutput 24 of diode bridge 22 is connected to the base of a first NPNtransistor Q1 through a zener diode 25 which serves as a threshold toeliminate any noise problems. The main electrodes of a triac 30 areserially connected to start winding 12. A resistor R2 is connectedbetween one main electrode of the triac and the gate thereof. A fullwave diode bridge rectifier 28 is connected across triac 30 thourghcurrent limiting resistor R1 and resistor R2 respectively. Diode bridge28 has a first output 26 connected to the collector of transistor Q1whose emitter is connected to line 34 extending from rectifier 22 torectifier 28. A second output of rectifier 28, line 32 is coupled toseries connected resistors R3, R4, and R5 in turn connected to line 34.Capacitor C1 is connected to a point intermediate resistors R3 and R4and to line 34. The base of a second NPN transistor Q2 is connected to apoint intermediate resistors R4 and R5 while the collector of transistorQ2 is connected to the output 24 of diode bridge 22 and the emitter isconnected to line 34.

When main winding 10 is energized, a pulse is generated by pulsetransformer 16 each time the current passes through zero. This pulse isrectified by bridge 22 and pulses transistor Q1 on which turns on bridge28 which in turn fires triac 30. Current goes through resistor R1, diodebridge 28, transistor Q1, back to the diode bridge 28 to the gate oftriac 30. If the pulse occurs after the start winding current crosseszero, as seen in FIG. 3, then the triac is fired for essentially a fullhalf cycle thereby energizing the start winding. As the motor speedincreases the phase angle between the main and the start winding currentchanges until at some point it changes sign and becomes negative withthe main winding current crossing zero before the start winding currentas seen in FIG. 5 at a point marked 75% of synchronous speed. This pointvaries from one type of motor to another but generally is between 65%and 85% of synchronous speed. When the pulse generated from the mainwinding current occurs before the start winding current crosses zero,the triac is essentially off the next half cycle as seen in FIG. 4 andthe voltage across triac 30 is high (line voltage, or in the case of acapacitor start motor even higher). This signal is rectified by bridge28 and charges capacitor C1 through resistor R3 which will turn ontransistor Q2 through resistors R4 and R5 to shunt current pulses awayfrom the base of transistor Q1 to thereby lock out triac 30. When triac30 conducts, there is only a volt or so drop across the triac however assoon as a high voltage appears across the triac, this signal is used tolockout the triac and hence the start winding so that the only way itcan be reenergized is to momentarily remove power from the circuit. Thislockout capability assures that an overload problem will not causerepetitive reenergization of the start winding which could causeoverheating and ultimately destruction of the triac.

One of the principle advantages of the present invention is that byproperly choosing the toroid transformer and motor rated triac, thecircuit can be applied to a class of motor types, for example fractionalhorsepower motors from a quarter to three quarters of a horsepower,without need for tailoring to a specific motor or type of motor. Whenused as a replacement starter, for example, one need only connect thestarter to the motor without having to consult the motor manufacturer todetermine what starter can be used.

A starter made in accordance with FIG. 1 had the following components:

    ______________________________________                                        winding 18  1T #20    Transistor                                                                              Q1  A5T5058                                   secondary 20                                                                              125T #34  Transistor                                                                              Q2  2N3904                                    diodes in rectifier 22                                                                    1N645     Capacitor C1  3.3 μf @ 25 V                          zener diode 25                                                                            1N748A    Resistor  R1  470 ohms                                              3.9 V                                                                                   Resistor  R2  220 ohms                                  diodes in rectifier 28                                                                    1N 4004   Resistor  R3  100K ohms                                 triac 30    TIC 246D  Resistor  R4  10K ohms                                                        Resistor  R5  3.3K ohms                                 ______________________________________                                    

Turning now to FIG. 2, it will be noted that winding 18 is placed in thesupply line 36 in order to generate pulses every time the line currentcrosses zero. As seen in FIG. 3, the slope of the line current,I_(LINE), at zero crossing is steeper than for I_(MAIN) so that asharper, narrower pulse is produced by the toroid thereby providing aneven more consistent control than is obtained in the FIG. 1 embodiment.It will also be noted from FIG. 3 that I_(LINE) current crosses zero ata point closer to the zero crossing of I_(START) current, generallyspeaking, 50% closer. This results in less torque lost during startwinding energization. Since the rest of FIG. 2 and its operation is thesame as for FIG. 1, the detailed description thereof will not berepeated.

In view of the above it will be seen that the present invention providesa more useful and reliable solid state starter than that of the priorart in that it has universal capability, i.e., it can be applied todifferent motor ratings and types without having to be tailored to aspecific application, it is independent of line voltage variations dueto its use of phase crossing to control firing of the triac, it has apositive lockout capability and through the use of the pulsetransformer.

It is within the purview of the invention to fire the triac directly bymeans of employing a larger toroid transformer to produce higher currentlevel pulses and use the energy derived from the main winding current orline current crossing zero to directly fire the triac if desired, ratherthan firing the triac by using the voltage across the triac as theenergy source.

It is to be understood that the specific embodiments of the inventionwhich have been described are merely illustrative and that numerousmodifications may be made by those skilled in the art without departingfrom the scope of the invention such as varistor protection ofcomponents against over voltage surges.

We claim:
 1. A control circuit for controlling the energization of thestart winding of an electric motor having a main winding and a startwinding both connectable with a source of a-c power comprising:pulsetransformer means for providing a pulse each time a reference a-ccurrent crosses zero; a solid state switch having two main electrodesand a gate electrode to control the conductivity of the switch, the twomain electrodes serially connectable to the start winding to controlenergization thereof; means responsive to the pulse coupled to the gateelectrode and the start winding for rendering the switch conductive foreach half cycle of start winding current only as long as the startwinding current passes through zero prior to the occurrence of thepulse.
 2. A control circuit according to claim 1 further including meansfor locking out the switch to preclude the switch from conducting whenthe start winding current passes through zero after the occurrence ofthe pulse.
 3. A control circuit according to claim 1 in which the pulsetransformer includes a coil connected serially to the main winding.
 4. Acontrol circuit according to claim 3 in which the pulse transformerincludes a tape wound toroid.
 5. A control circuit according to claim 1in which the means responsive to the pulse includes a first transistor,the pulse transformer means including a rectifier for rectifying theoutput of the transformer means, the rectified pulse fed to the base ofthe transistor, the emitter and collector of the first transistorcoupled to the gate electrode of the switch.
 6. A control circuitaccording to claim 5 in which the first transistor is an NPN type andfurther including another rectifier, the rectifier having an output, thecollector of the first transistor connected through the output of therectifier to a point intermediate the start winding and the switch.
 7. Acontrol circuit according to claim 6 including a second transistor, thebase of the second transistor coupled to the output of the said anotherrectifier and adapted to turn on the second transistor when the voltageacross the solid state switch is high, the collector, emitter circuit ofthe second transistor being connected to the rectifier of the pulsetransformer and adapted to shunt the pulses away from the firsttransistor when the second transistor is turned on.
 8. A control circuitaccording to claim 7 further including an R-C circuit coupled betweenthe output of the said another rectifier and the base of the secondtransistor.
 9. A control circuit according to claim 1 in which the solidstate switch is a triac.
 10. A control circuit according to claim 2 inwhich the solid state switch is a triac.
 11. A control circuit forcontrolling the energization of the start winding of an electric motorand for locking out the start winding upon deenergization thereof, themotor having a main winding and a start winding both connectable with asource of a-c power comprising:signal means for providing a signal eachtime a reference a-c current crosses zero; a triac having two mainelectrodes and a gate electrode, the two main electrodes seriallyconnectable to the start winding to control energization thereof; meansresponsive to the signal coupled to the gate electrode and the startwinding for rendering the triac conductive for each half cycle of startwinding current as long as the start winding current passes through zeroprior to the occurrence of the signal; lockout means to preclude triacconduction when the start winding current passes through zero after theoccurrence of the signal including a transistor having acollector-emitter circuit coupled to the signal means and adapted toshunt the signal away from the said means responsive to the signal whenthe transistor is turned on, and voltage responsive means coupled to thetriac and the base of the transistor so that when there is a highvoltage across the triac, the transistor will be turned on to therebylock out the triac.
 12. A control circuit according to claim 11 in whichthe transistor is an NPN transistor.
 13. A control circuit according toclaim 11 in which the voltage responsive means includes an RC circuit.14. A control circuit according to claim 11 in which the reference a-ccurrent is the main winding current.
 15. A control circuit forcontrolling the operation of an induction motor having a start windingand a main winding energized by an alternating current energy source andfor deenergizing the start winding in response to a phase crossingbetween the main winding current and the start winding currentcomprising,a three terminal solid state switch means having a pair ofterminals connectable in series circuit relationship with the startwinding and a gate terminal, pulse generating means connectable to themain winding for producing a narrow pulse each time the main windingcurrent crosses zero, trigger circuit means for energizing the solidstate switch means, the trigger circuit means having a first transistorcoupled to the gate of the solid state switch and adapted to energizethe switch when the transistor is conductive, the pulse generating meansconnected to the first transistor so that a pulse transmitted from thepulse generating means to the first transistor will render itconductive, lockout circuit means including a second transistorconnected in parallel circuit relationship with the first transistor andmeans responsive to a preselected voltage across the solid state switchmeans to turn on the second transistor to shunt the pulses from thepulse generating means away from the first transistor thereby preventingthe triggering of the solid state switch and deenergizing the startcircuit.